In the conventional art, fluorescent tubes such as cold cathode tubes are used for a backlight apparatus constituting a liquid crystal display. Generally, a cold cathode tube is configured such that a phosphor is applied to the inner wall of a cylindrical glass tube and inert gas (such as argon) and mercury are contained in the glass tube. Further, a high voltage is applied across electrodes on both end portions of the glass tube to start discharge, and the evaporated mercury is excited by collisions with electrons and the atoms of the filler gas and generates ultraviolet rays. The ultraviolet rays excite the phosphor applied to the inner wall of the glass tube and generate visible radiation typified by white light.
In a cold cathode tube configured thus, the electrodes provided on both end portions may be cylindrical. In this case, when the cold cathode tube is used over an extended period, a sputtering phenomenon occurs on the electrodes, particularly on the edge portions of the cylinder. Thus, mercury in the tube may be taken into a sputtered layer formed by the sputtering, so that the luminous efficiency and life of the cold cathode tube may be reduced.
For example, a technique described in patent document 1 has been known as means for suppressing the sputtering. A cold cathode tube described in patent document 1 is configured such that a cap made of an electrical insulating material having high heat resistance is provided on the end portion of an electrode closest to an ionization region. With this configuration, it is possible to reduce or suppress the occurrence of sputtering on the end, so that the life of the cold cathode tube can be increased.
[Patent Document 1] National Publication of International Patent Application No. 2007-506228
In recent years, the number of cold cathode tubes arranged in a backlight apparatus has been increased in response to the need for larger liquid crystal displays, thereby increasing the cost. Thus, it has been examined that the number of used cold cathode tubes is reduced with a constant luminance by increasing the diameters of the cold cathode tubes. When cylindrical electrodes according to the conventional art are used in a cold cathode tube having a large diameter, the following problems may occur:
For example, when the cylindrical electrode has a diameter as in the conventional art, there is a large gap between a side of a glass tube and the electrode. Thus, accelerated electrons reaching the electrodes are not contained in the cylinder and scatter around the electrode, so that sputtering occurs. Consequently, the life of the cold cathode tube may be reduced.
On the other hand, when the cylindrical electrode is increased in diameter, accelerated electrons intensively collide with the bottom face of the cylinder. Thus, sputtering locally occurs on an electrode material or an emitter material on the bottom face, so that the life of the cold cathode tube may be reduced.